Risk: Looking Forward
Risk generally refers to the probability of some untoward event. In this chapter, the term risk is used in a more restricted sense to indicate the probability that people who are exposed to certain "risk factors" will subsequently develop a particular disease more often than similar people who are not exposed
This chapter describes how investigators obtain estimates of risk by observing the relationship between exposure to possible risk factors and the subsequent incidence of disease. To describe methods that determine risk by following groups into the future.To discuss several ways of comparing risks as they affect individuals and populations
Characteristics associated with an increased risk of becoming diseased are called risk factors. Exposure to a risk factor means that a person, before becoming ill, has come in contact with or has manifested the factor in question.Exposure can take place at a single point in time, as when a community is exposed to radiation during a nuclear accident.Contuct with risk factor for chronic disease
For example, having the haplotype HLA-B27 greatly increases one's risk of acquiring the spondylarthropathies. Work on the Human Genome Project has identified many other diseases for which specific genes are risk factors, including colon and breast cancer, osteoporosis, and amyotropic lateral sclerosis. Other risk factors, such as infectious agents, drugs, and toxins, are found in the physical environment
Still others are part of the social environment. For example, bereavement after the loss of a spouse, change in daily routines, and crowding all have been shown to increase rates of disease-not only emotional illness but physical illness as well. Some of the most powerful risk factors are behavioral; examples are smoking, drinking alcohol to excess, driving without seat belts, engaging in unsafe sex, eating too much, and exercising too little.
Exposure to a risk factor means that a person, before becoming ill, has come in contact with or has manifested the factor in questionExposure can take place at a single point in timecontact with risk factors for chronic disease takes place over a period of timeAlthough the various measures of dose tend to be related to one another, some may show an exposure-disease relationship, whereas others may not
RECOGNIZING RISKLarge risks associated with effects that occur rapidly after exposure are easy for anyone to recognize:Chikenpox, sunborn becauseThese conditions follow exposure relatively rapidly and with obvious effects.Most morbidity or mortality is caused by chronic diseases for which the relationships between exposure and disease are far less obvious
Long LatencyMany chronic diseases have long latency periods between exposure to a risk factor and the first manifestations of diseaseWhen patients experience the consequence of exposure to a risk factor years later, the original exposure may be all but forgotten and the link between exposure and disease obscured
Common Exposure to Risk FactorsMany risk factors, such as cigarette smoking or eating a diet high in cholesterol and saturated fats, have become so common in western societies that for many years it was difficult to discern their dangers.Using cross-national studies or investigating special subgroups- Mormons
Low Incidence of Disease
The onset of most diseases. even ones thought to be "common," is actually uncommon. Thus, although lung cancer is the most common cause of cancer deaths in Americans, the yearly incidence of lung cancer, even in people who have smoked heavily for 25 years, is 2 to 3 in 1000To draw conclisions about risks from such infrequent events.
Most chronic diseases are caused by several risk factors acting together. The risk associated with any one of them, taken alone, is small. To detect this risk, a large number of people must be studied to observe a difference in disease rates between exposed and unexposed persons. This is true even when both the risk factor and the disease occur relatively frequently
When a disease is common such as heart disease, cancer, or stroke and some of the risk factors for it are already known, it becomes difficult to distinguish a new risk factor from the others.There may be less incentive to look for new risk factors
Multiple Causes and Effects
There is usually not a close, one-to-one relationship between a risk factor and a particular diseaseThere relationship between hypertension and congestive failure For all these reasons, individual clinicians are rarely in a position to recognize, let alone confirm, associations between exposure and chronic diseasesThe medical literature
USES OF RISK
In clinical medicine, knowledge of risk factors can be used in several different waysRisk Factors Predict Future DiseaseRisk Factors May or May Not Be CausalRisk Factors Help Establish Pretest DiseaseProbability for Diagnostic TestingRisk Stratification for Screening ProgramsRemoving Risk Factors May Prevent Disease
Risk Factors Predict Future Disease
Risk factors are used, first and foremost, to predict the occurrence of disease. The best available information for predicting disease in an individual person is past experience with a large number of people who have a similar risk factor. How well the prediction applies to the individual depends on the similarity of the individual person to the group and is, in any case, never perfect.
Risk Factors May or May Not Be Causal
The search for risk factors usually is a search for causes of disease. In clinical medicine, physicians are more comfortable with immediate causes of disease infectious, physiologic, or anatomic changes leading to sickness such as a coronavirus causing SARS or hypocalcemia leading to seizures. But distant causes, more remote from a condition, may be important in the causal pathway.Low birth- weight infants
Risk Factors May or May Not Be Causal
It is important to remember that just because risk factors predict disease, it does not necessarily follow that they cause disease. A risk factor may predict a disease outcome indirectly, by virtue of an association with some determinant of diseaseA risk factor that is not a cause of disease is called a marker of disease, because it "marks" the increased probability of disease
Risk Factors Help Establish Pretest DiseaseProbability for Diagnostic TestingKnowledge of risk can be used in the diagnostic process, since the presence of a risk factor increases the pretest probability of disease among patients, which is one way of improving the positive predictive value of a diagnostic test. However, in individual patients, risk factors usually are not as strong a predictor of disease as are clinical findings of early disease
The absence of a very strong risk factor may help to rule out disease. Thus, it is reasonable to consider mesothelioma in the differential diagnosis of a pleural mass in a parient who is an asbestos worker. But mesothelioma is a much less likely diagnosis for the patient who has never been exposed to asbestos.
Risk Stratification for Screening Programs
Knowledge of risk factors can be used to improve the efficiency of screening programs by selecting subgroups of patients at substantially increased risk. screening for colorectal cancer is recommended for the general population starring at age 50. However, people with a first-degree relative with a history of colorectal cancer are at increased risk for the disease, and several expert groups suggest that screening these people should begin at age 40.
Removing Risk Factors May Prevent Disease
If a risk factor is also a cause of disease, removing it can prevent disease whether or not the mechanism by which the disease develops is known. Some of the classic successes in the history of epidemiology illustrate this point. ChleraHIV
STUDIES OF RISK
The most powerful way to determine whether exposure to a potential risk factor results in an increased risk of disease is to conduct an experiment in which the researcher determines who is exposed. People currently without disease are divided into groups of equal susceptibility to the disease in question. One group is exposed to the purported risk factor and the other is not, but the groups otherwise are treated the same
When Experiments Are Not Possible
The effects of most risk factors in humans cannot be studied with experimental studies. Consider some of the risk questions that concern us today:Are inactive people at increased risk for cardiovascular disease, everything else being equal? Do cellular phones cause brain cancer? Does a high-fat diet increase the risk of breast cancer?it is usually not possible to conduct an experiment.Unethical, having their diets and behaviors, difficult and expensive
Clinical studies in which the researcher gathers data by simply observing events as they happen, without playing an active part in what takes place, are called observational studies. Most studies of risk are observational studies and are either cohort studies
Cohortsthe term cohort is used to describe a group of people who have something in common when